This invention relates to computer-to-plate (CTP, C2P) imaging for flexographic (“flexo”) printing, and in particular, to a method and apparatus for loading and unloading flexographic plates onto an imaging device for imaging the flexographic plates.
Flexographic plates designed for CTP imaging are typically photopolymer plates that are pre-sensitized with a Laser Ablation Mask System (LAMS) coating. Such coatings are known to be easily damaged during handling. Pre-sensitized plates, including flexo-plates for flexographic CTP imaging come as a stack with a separation sheet between consecutive plates. In addition, LAMS coated flexographic plates come with a cover sheet that adheres to the top of the LAMS coating to protect it against mechanical damage. This protective sheet is difficult to remove without damaging the LAMS coating or the plate itself. In addition, such plates can be relatively large, up to about 50″ by 80″, with a thickness of up to about 6 mm. Each such plate can weigh as much as 15 kg.
These properties make plate loading for CTP flexographic imaging relatively complicated. As a result, modern flexographic CTP imagers are manually loaded and unloaded. Plate loading tables are available that help the manual process. Such tables are either movable, e.g., on wheels, or are integrated into the machine itself. Each plate, e.g., pre-loaded on the loading table, is carried individually from the plate storage area to the CTP flexographic imager, and, after imaging is completed, carried further onwards, e.g., to a processing area where to be processed by a processing system, e.g., exposed by a UV exposure unit then processed by a chemical processing unit.
The requirement to move the plates one-by-one significantly slows down the workflow. With a typical state-of-the art system using a transportable loading table, for a large plate, it may take about 2 minutes to load a first plate, about 10 minutes to image the first plate, about 2 minutes to unload the first plate, about 5 minutes to carry the first plate to the UV exposure unit of the processing system, then 2 minutes to load a second plate, and so forth. Thus, for 10 minutes imaging time, there is about 9 minutes of downtime. This is approximately 45% of the total time.
The efficiency can be increased if more than one operator is involved, e.g., a second operator to transport and load the second plate while the first plate is being transported to the processing area. However, an additional operator is relatively expensive.
There thus is a need to improve the overall productivity of flexographic plate imaging, especially in a single operator environment, by improving the loading and unloading.
FIG. 2 shows a prior art CTP imaging system that is described in U.S. Pat. No. 6,341,932 to Otsuji titled PLATE FEEDING APPARATUS AND METHOD, incorporated herein by reference, and referred to herein as the “Otsuji system.” The Otsuji system comprises a plate feeding apparatus 2 that includes a multiple cassette station 5 having a plurality of cassettes 7 arranged one over the other, each cassette holding a stack of plates. The Otsuji system also comprises a loader 6 that includes a slide mechanism for horizontally moving a particular one of the plurality of cassettes from the stack to the loader and a lift mechanism for supporting and vertically moving the particular cassette 7 to a plate feed position. A transport mechanism in loader 6 transports a plate from the particular cassette to the image recording apparatus (imager) after the particular cassette is at the plate feed position. The imager is not shown in FIG. 2, but is behind the two mechanisms 5 and 6 so that feeding a plate involves moving a plate into the plane of the page. The loader 6 includes a slip sheet discharge mechanism that picks up and discharges slip sheets each disposed between an adjacent pair of the plates in the particular cassette 7 at the feed position.
FIG. 3 is taken from U.S. Pat. No. 5,738,014 to Rombult, et al. titled METHOD AND APPARATUS FOR MAKING LITHOGRAPHIC PRINTING PLATES IN AN AUTOMATED COMPUTER TO PLATE IMAGING SYSTEM, and incorporated herein by reference. FIG. 3 shows a CTP imaging system 16, referred to herein as the “Rombult system” that includes a plate handler 18 that contains a supply of plate cassettes 24. The handler 18 can hold as few as two cassettes or as many as three, four, or five depending on user requirements. Each cassette 24 is a light tight container that houses a stack of plates 26, typically lithographic plates. The cassettes 24 can be vertically adjusted by the handler 18 to make plates 26 stored within a particular cassette available to a plate shuttle mechanism (a plate picker 28). The picker 28 removes a single plate from the stack in the selected cassette and transports the plate between the handler 18 and an imager 20. The primary function of the handler 18 is to make plates available on demand to the imager 20. Between each plate in a stack there may be a protective interleaf sheet or slip sheet that is removed by the handler and discarded by a slip sheet removal mechanism 25. The Rombult system 16 includes an optional on-line plate processor and stacker to process the plates after exposure by the imager 20. The Rombult system 16 is controlled by a controller 30.
Each cassette in the Otsuji system and the Rombult system stores a stack of a plurality of pre-sensitized plates with a slip sheet between the plates. One use of the Otsuji system and the Rombult system is for each cassette to store a stack of plates of a different size and/or different thickness so that different size/thickness plates are always available to the imager.
The present invention further addresses a different problem. In practice, it is very difficult to stack flexographic plates. The operation of removing the protective sheet on top of the LAMS coating is a relatively delicate operation not readily automated. Thus a system such as the Otsuji system or the Rombult system wherein each cassette includes a plurality of plates may not be suitable for flexographic plates. The Rombult system patent acknowledges that it is for lithographic CTP imaging for the purpose of supplying the plates of the appropriately sized cassette on demand, and is shown operating with an internal drum scanner. Size is also an important consideration. The Otsuji system's plate feeding apparatus 2 includes a cassette station 5 and a loader 6 that each requires about the same floor area.
Thus there still is a need for a method and apparatus to aid in the loading and unloading of LAMS-coated flexographic plates for CTP imaging.
Flexographic plates can be relatively large, so that there is a need for a loading method and apparatus for flexographic plates that is economical in floor area requirements.